Voltage halver.

Something like this would do the job, provided you're careful to limit the voltage you use!

This is a cheap variable bench power supply which will supply 5A at 6V. You may want to look at your local market for a suitable mains voltage and plug, and there are a lot of makes.

Otherwise, AFAICS you're going to need a somewhat more than 6V 5A power supply and a separate variable voltage converter, which won't be much less in price, and may need a box to put it in for electrical neatness.

BTW, my memory of incandescent bulbs is that at 1.5 volts a 6V bulb will barely glow a dim red, BICBW.

A 6v transformer and an adjustable resistor made from an old electric fire bar and two jubilee clips?

I thought of that, but it is not very useful to adjust while looking down the microscope and I wondered if AC might affect microphotography. Have you seen the price of 30W WW pots or "rheostats"?

Also, an electric fire element is going to be about 30ohms in North America, and since you want fractions of 2ohms adjustment over 1 to 3 turns is going to be somewhat fiddly.

I hadn't, but I expect they would be expensive.. By comparison, making big power resistors is surprisingly cheap -- I had to make some for a

5kW organ blower starter, so I had some slate bars cut for me by a monumental mason and wound them by hand (with tappings). The total cost was surprisingly low.

I don't see how AC wold be a problem with microphotography unless you were worried about ripple in the light output. Low voltage lamps like that heve a short thick filament with high thermal inertia, so they were often used as the exciter lamp for for sound film reproduction. Any ripple would have generated a 100c/s hum, but there has been very little sign of that on the vintage projectors I have worked on.

Certainly adjustment while looking down the scope must be convenient.

Input voltage is another factor I should have mentioned. The scope is going to Africa where the wall socket will probably be 220 V. The 3-12 V adapter I have accepts 110-240 V input. Good.

6 V is roughly the bottom 1/8 th of the range of the adapter. Halving the output with efficient switching would allow use of the full range of adjustment.

I was hoping someone would cite a voltage halver as simple as this.

Soldering leads to an IC would be a reasonable 2nd possibility.

Thx, ... P.

If you connect two identical bulbs in series, then the voltage across each will be half of the supply voltage. So, connect another identical bulb in series. You won't find a simpler circuit!

What does the microscope manufacturer supply for this function?

If you have a >6V source, simple 555-type buck regulators are available with minimum output listed as 1.8V

These generally regulate the negative output terminal, unfiltered. This would not be an issue for an isolated 6V incandescent lamp.

Rl

Understood. Thanks. No spare bulb handy but can put a resistor in series with the bulb.

Switched voltage halving would be more efficient.

A 6 V, 5 A adapter similar to eBay 305207182573 would be even better but I haven't found one.

Thanks, ... P.

The scope is a Zeiss OPMI 1. Google finds many listings of various models with various power supplies.

This scope was donated without the power supply; I can't say what the original supply was but Zeiss built nice equipment capable of continuous and reliable duty.

Good idea. Will see what Lee's Electronics has in stock. I expect to have to limit the voltage somehow to prevent burning the bulb.

Thanks, ... P.

Using a lower voltage wall wart as a source is the easiest limiting method. Overvolting the bulb by 50% temporarily will not be catastrophic. If the wall wart is internally limited for power or current, then overvoltage is unlikely.

There may be issues with wall wart curremt limiting which is best addressed with filtering (~ capacitive) between the two devices.

The 555 buck regulators are dumb, in themselves, which is often an advantage driving nonlinear (incandescent) loads. The unit with the large yellow PTC on-board is a good bet as load protection.

Just remeber that the 'common' rail in the system will be the +rail of the 555 buck output. The wall wart won't care - neither will the lamp. You're effectively driving the lamp with a negative supply.

The unfiltered output is a high frequency square wave with peaks equal to the wall wart's labelled voltage.

Filtering that output to DC would require an external 5A buck diode, a 5A choke and an electrolytic cap - effectively doubling the size, weight (and cost) of the regulator section, at no benefit to the lamp.

RL

If you are intending use of this device in it's intended environment, you should use the mfr's designated power supply, as this is the major system component that gives it biomedical safety certification.

RL

I also note that in the zeiss opmi_1 manual, a 12V 100W halogen lamp is specified - and a self-resetting thermal limiter is built into the lamp holder.

RL

Put a few diodes in series. More diodes and less voltage of about .5 to .7 volts per diode.

Temu has an enclosed 0-12v LED dimmer for $2.69 that might work with a wall wart but only "rated" for 8A.

The zeiss opmi_1 uses a light pipe.

RL

You have an original paper manual?

OPMI 1 is a family of microscopes rather than a specific model; correct?

A search in eBay, for example, finds many variants. Appears that components were assembled according to requirements of a customer.

A search found this URL.

The 2nd supplementary page after page 31 lists these bulbs.

Endure Number Spare Bulbs

90-1200 Zeiss 6V 30W Bt58Z 390158 90-1201 Zeiss 6V 50W Bt86Z 390186 90-1202 Zeiss 12V 100W HLX #64626 380075 1020 90-1203 EFR Housing #900 90-1204 Zeiss 15V 150W EFR 310198 90-1205 Zeiss 12V 100W HLX #64627 380079 9040 90-1206 Zeiss Superlux 40 90-1207 Zeiss Superlux 175 90-1208 Zeiss Superlux 300 with Cartridge 90-1209 Zeiss Superlux 300 Bulb Only - No Housing or Meter 90-1302 ELS 150 21V 150W EKE 90-1400 ELS 250 24V 250W ELC 90-1403 ELS 24 60V 24W Metal Halide 90-1402 ILO 300W with Cartridge

EFR, ELS and ILO are bulb manufacturers?

After my original enquiry, also found a page for a replacement LED illuminator. 700+ $US. Beyond my budget at present.

Also found some old bulbs in boxes labeled 390153. They have the unusual (Zeiss?) triangular base and the filament is parallel to the bulb axis. The 39015 has the filament crosswise to the bulb axis.

Also found an inexpensive motor controller which adjusts light intensity nicely. The adjustable power brick can be set to deliver 6 V. The controller connected beyond that adjusts intensity.

In case a 90-1202 Zeiss 12V as listed above turns up, my power supply should suffice for that also.

Thanks to all for the replies, ... P.

In the reference I found for zeiss opmi_1, illumination is delivered to the optics package through a long (1m ?) light pipe (or possibly bundles of same).

RL